1. Field of the Invention
This invention relates to internal combustion engine systems with multi-stage turbocharging systems, and the use of multi-stage turbines in general.
2. Description of the Related Art
Turbocharging systems, such as for use with internal combustion engines, are well-known in the art. A turbocharger comprises an exhaust gas turbine coupled to a gas intake charge compressor. The turbine operates by receiving exhaust gas from an internal combustion engine and converting a portion of the energy in that exhaust gas stream into mechanical energy by passing the exhaust stream over blades of a turbine wheel, and thereby causing the turbine wheel to rotate. This rotational force is then utilized by a compressor, coupled by a shaft to the turbine wheel, to compress a quantity of air to a pressure higher than the surrounding atmosphere, which then provides an increased amount of air available to be drawn into the internal combustion engine cylinders during the engine's intake stroke. The additional compressed air (boost) taken into the cylinders can allow more fuel to be burned within the cylinder, and thereby offers the opportunity to increase the engine's power output.
In a turbocharged internal combustion engine system, the wide range of speed and power output levels at which the internal combustion engine may operate presents challenges for designing an appropriately matched turbocharging system with good mechanical efficiency for working with the engine. For example, while smaller turbochargers provide boost quickly and more efficiently at lower engine speeds, larger turbochargers provide boost more effectively at higher engine speeds. Because of the relatively narrow flow range over which a turbocharger operates efficiently, relative to the broader flow range generated by internal combustion engines, it is known in the prior art (e.g., in cases of high boost need), to provide a multi-stage turbocharging system, involving both a smaller (i.e. “high pressure”) turbocharger and a larger (i.e. “low pressure”) turbocharger, wherein the smaller high pressure turbocharger operates at lower engine speeds and the larger low pressure turbocharger takes over at higher engine speeds. It has been found valuable to switch between the two turbocharging stages through use of a bypass system to divert exhaust gas flow around the higher pressure turbocharger to the lower pressure turbocharger as needed.
As a result, bypassing exhaust flow around a turbine gas expander is also well-known in the art. Typically, turbine bypass systems are used in the prior art primarily to regulate system pressure across the higher stage turbine wheel, and can be operated by selectively bleeding off a portion of the upstream exhaust gas over a pressure drop through a bypass channel when backpressure caused by the turbine's operation causes the system pressure upstream of the turbine to exceed desired levels. Bleeding of the exhaust gas through the bypass channel is generally controlled by a small regulating valve (called a “wastegate”) in the exhaust piping channel around the turbine. A typical wastegate valve operates somewhat like a trap door, opening a port from the higher pressure turbine inlet to a lower pressure area by diverting a portion of the exhaust flow through a bypass channel around the turbine, with the bypassed exhaust flow naturally expanding over the pressure drop in the bypass channel and then reuniting with the remaining exhaust flow downstream of the bypassed turbine.